The present invention relates to the spraying of liquids and, in particular, to the electrostatic spraying of liquids.
Spraying apparatus is known in which a high velocity air stream is used to atomize a liquid issuing from a nozzle for subsequent deposition on a crop or plant.
Such a device is shown in U.S. Pat. No. 3,504,854 to R. J. A. DeKinkelder where liquid is dispensed from a plurality of nozzles located within a flared outlet duct. The duct is supplied with a high velocity air stream which atomizes and entrains liquid being dispensed from the nozzles and carries the liquid into the atmosphere in the region of the crop being sprayed. This device has been commercially exploited and provides an improvement over previously known devices.
As an improvement of the DeKinkelder apparatus, the droplets are charged electrostatically as they pass through the outlet duct and thereby improve the deposition of the droplets on the crop. In Canadian Pat. No. 1,114,427 which issued on Dec. 15, 1981, to I. I. Inculet and G. S. P. Castle, a system is described wherein an electrode is placed in the duct facing the nozzles and connected to one terminal of a high voltage source. The other terminal of the source is connected through the apparatus to ground. An electric charge is induced on the droplets as they are formed in the duct. The charged droplets are attracted electrostatically to the leaves of the crop being sprayed. This has improved the deposition of liquid droplets on the crop and increased the efficiency of the spraying apparatus.
However, since electrostatically charged droplets in a cloud tend to repel one another, the cloud will expand both vertically and horizontally. Though, as described in the publication "Space Charge Effects in Orchard Spraying" by G. S. P. Castle and I. I. Inculet, in Conf. Rec. 1981, 16th Annual Meeting, IEEE Ind. Appl. Soc., pp. 1155-1160, it has been shown that the electrostatically charged liquid droplets within the cloud produced by the sprayer are attracted to ground potential, i.e. the ground cover such as the trees, leaves and other vegetation and earth, at the same time, the droplets repel one another. At a certain strata in the vertical direction in the cloud, these forces are in equilibrium so that droplets below this strata have a net force vertically down and droplets above this strata have a net force vertically upward. The droplets above the equilibrium strata, therefore, tend to be dispersed and drift to adjacent areas. This drifting is both wasteful of the chemicals being sprayed and potentially hazardous to surrounding areas.
In the horizontal direction, the droplets are also attracted to ground potential exhibited by the trees, leaves, etc. The droplets also repel one another causing the cloud to expand. Once again, one side of the cloud will tend to expand away from the intended object to be sprayed. However, in large fields or orchards this loss would be less severe since the spray would usually drift to the next row.